SVQ3: Fix decoding with A32_BITSTREAM_READER

svq3_decode_slice_header() modifies the buffer used by the bitstream
reader.  Some of the bitstream readers cache a few bytes of data, which
must be flushed after such a modification.  Calling skip_bits_long(gb, 0)
achieves this.

Originally committed as revision 17680 to svn://svn.ffmpeg.org/ffmpeg/trunk